High-density　polyethylene　(HDPE)　pipelines　are　widely　used　for　the　transportation　of　natural　gas.　The　butt-fusion　welded　joints　melt　and　cool　during　the　welding　process,　resulting　in　changes　in　mechanical　properties,　molecular　chain　spatial　position　microstructure,　and　functional　groups.　Herein,　we　investigate　the　aging　behavior　of　an　HDPE　butt-fusion　welded　joint　in　accelerated　thermal-oxidative　aging　tests　under　various　temperature　gradients.　The　Vicat　softening　temperature,　oxidation　induction　time,　and　infrared　spectrum　were　measured,　and　the　microstructures　were　observed.　The　results　indicated　that　the　mechanical　and　chemical　properties　of　the　butt-fusion　welded　joint　degraded　with　incresing　aging　temperature.　Analysis　was　conducted　to　identify　the　molecular　chain　intersection　mechanism　in　the　heat-affected　zone　and　the　weld　joining　mechanism.　The　findings　help　understand　the　aging　behavior　of　HDPE　and　provide　guidelines　to　reduce　the　risk　caused　by　butt-fusion　welded　joint　degradation.